The present invention relates to optical time division multiplexed communication systems, and in particular, to a node for use in such systems, and to a method of operating a node for such systems.
Telecommunications operators, in order to meet the demand from their customers for increasingly sophisticated and bandwidth intensive services, require networks which are reconfigurable, and which operate at higher data rates than those installed to date. Optical transmission links have been utilised in telecommunications networks to increase the data carrying capacity of the network from one point to another point. It is known to employ OTDM (optical time division multiplexing) to share the bandwidth available on the optical transmission link between several channels by interleaving the channels in time. To increase the flexibility of the communications network, provision is often made, at nodes of the network, to extract, or "drop", a channel or channels at a node, or to add, or "insert", a channel or channels at a node. Both for point to point OTDM systems, and for OTDM networks, if the processing (ie. multiplexing, de-multiplexing, drop, and insert) is carried out electronically, the data carrying capacity of the communications system will be limited by the operating speed of the electronic components. Thus, rather than converting optical signals to electronic signals at each node of the network, carrying out any necessary processing on the signals electronically, and reconverting the resultant electronic signals to optical signals for onward transmission, it is desirable to perform the processing functions optically so as not to restrict the bandwidth of the OTDM system.
Many of the experimental OTDM systems employing optical processing that have been reported, for example by D M Spirit and L C Blank in "Optical Time Division Multiplexing For Future High Capacity Network Applications", BT Technology Journal Vol. 11 No. 2, April 1993, interleave optical channels which have originated from the same single optical source. However, when practical OTDM systems are considered, there is a high likelihood that each optical channel will originate from a different optical source, so that the optical signal leaving a node of the network contains interleaved pulses from a number of distinct optical sources. If these numerous optical sources do not produce substantially identical, high quality optical pulses the error rate of the OTDM system may become unacceptable upon subsequent transmission. Furthermore these experimental OTDM systems often comprise single point to point optical links.
For an OTDM network, as opposed to a point to point link, a further problem arises because each optical channel received at a particular node of the network may have travelled a different distance. Thus in an OTDM network, even if all the optical sources produce identical optical pulses, or a single optical source is employed for the whole network (as suggested in Spirit and Blank), the outgoing optical pulses from a particular node will have different optical properties. The outgoing optical pulses may, for example, be of differing widths due to dispersion suffered during transmission over previous links of different lengths. The pulses are attenuated during transmission and the changes in pulse width make it difficult to match the local launch power to that of the received pulses, since the ratio of mean to peak power varies with pulse width. These differing optical properties of the pulses may cause difficulties both within the node (eg. affecting the receiver sensitivity and switching efficiency), and in their subsequent propagation in the OTDM network where the differing optical properties of the pulses may cause them to evolve in a different manner leading, for example, to time slot errors. These effects would be cumulative as further nodes of the OTDM network are encountered, and would be exacerbated in a practical system where numerous, non-identical, optical sources are likely to be employed.